Really liked the demo! Three things stuck out for me:
1. When I switched off +/- markers in the menu, it only switched them off of the magnet and pipes; they stayed on for the magnetic fields.
2. I eventually got used to picking up the key ending the level, but it was jarring the first time for me, if only because I'm used to picking up keys partway through a level. It's not a big deal, but it was a bit jarring the first time I picked up a key and the level switch happened.
3. Like with most folks, the last level was a bit of a struggle for me. Largely because the first level involved using a magnetic field to throw the magnet, I unsuccessfully struggled with the first two methods Erakos mentioned before finally figuring out the intended solution. I really like the final solution, but there was a period of time where I was frustrated not knowing if I had run across the intended solution or a wokaround.
Besides that, I thought it was a lot of fun! I really like that you included being able to click the left mouse to throw the magnet - it made picking up and throwing things much easier from me than aiming with the mouse and pressing a keyboard button. I had no difficulty with the mouse/keyboard setup, but I'm used to using WASD for computer games, rather than the arrow keys.